Add synthetic JavaBean getters to AutoValue classes
This PR adds an AutoValue extension based on https://github.com/Graylog2/graylog2-server/issues/3127#issuecomment-265772246 which generates synthetic getters following the JavaBean conventions.
It also adds the new `@WithBeanGetter` annotation to the relevant AutoValue classes.
## Types of changes
- [x] Bug fix (non-breaking change which fixes an issue)
- [x] My code follows the code style of this project.
- [x] I have read the **CONTRIBUTING** document.
- [x] I have added tests to cover my changes.
- [x] All new and existing tests passed.
Looks good to me so far. Let me know once it's ready for a final review.